1. Field of the Invention
The present invention relates to a method of repair a scattering stencil type mask utilized in the electron-beam projection lithography systems.
2. Description of the Related Art
In manufacture of semiconductor elements such as IC and LSI, a lithography process is used to create fine circuit patterns on a semiconductor substrate. The performance of the semiconductor elements is mostly determined by the number of circuits provided in each element, which significantly dependent on the size of patterns of circuits. The recent developments in semiconductor integrated circuit production technology have been remarkable, and show a strong tendency toward miniaturization and further integration. As a method of forming an integrated circuit pattern on semiconductor substrates, a lithography method utilizing ultraviolet light is commonly used.
However, with the further miniaturization of the circuit pattern, it has been a concern that the resolution of the light is approaching its limit. Thus, the high-resolution lithography technology using a charged particle beam such as electron beam and ion beam, or X-ray is being examined. For example, since exposure technology utilizing a charged particle beam can make the beam diameter to be as small as order of nm, it is characterized by that a miniaturized pattern below 100 nm can be easily formed, and an electron beam writing technology has been known for a long time. However, in so-called direct-writing method where the writing is performed by scanning very fine electron beam, it takes a very long time to form a large-area or large-size pattern, that is, the throughput (a processing rate per unit time) is low. For that reason, a photolithography method where the light source is ultraviolet light is still being used as the lithography method of manufacturing the semiconductor integrated circuits, while the electron beam direct-writing method is used only in limited fields such as in manufacturing of reticles, or masks, for use with photolithography and trial manufacturing of devices for the purpose of experimentation.
Then, in order to solve such a problem, lithography technology EBPS (Electron Beam Projection System) is proposed, in which, instead of a direct writing by a charged particle, a predetermined reticle pattern is transferred by reduction-projection on a wafer by means of an electron optical system.
For example, Jpn. J. Appl. Phys. 34 (1995), page 6658, explains the electron beam lithography technology in terms of its development from the direct writing method using an electron beam as a projection light source, to a method of projection exposure with variable axis immersion lenses.
As a reticle used in the EBPS, it seems that a scattering stencil type thereof is desirable in light of the exposure characteristics. The scattering stencil type includes a pattern portion to be exposed which consists of voids or openings and a supporting structure, membrane. The method for forming a scattering stencil type reticle is such that an oxide film is formed on a silicon substrate and then a predetermined pattern is formed on this oxide film. The silicon substrate is then etched in accordance with this pattern.
For this scattering stencil type reticle, a pattern defect is always caused at the time of forming the reticle. Namely, there are occasions where the material is absent in the necessary electron beam scattering material pattern (clear defect), or an extra material is occurred due to an additional amount of electron beam scattering material (opaque defect). In general, it is considered that the clear pattern is caused by an error made at the time when the pattern is projected onto a membrane (at the time of the resist-pattern writing), so that the portion where the resist pattern is missing becomes itself a clear defect. On the other hand, a opaque defect is the electron beam scattering material which has remained in unwanted areas due to the fact that such foreign materials or the like, for example one that could become an etching mask, adheres to the resist pattern.
As a method which repairs a opaque defect, it is possible to undertake selective milling by irradiating a charged particle beam such as a focused ion beam so as to perform pattern correction thereof. However, there is a problem that after the milling is once performed the material adheres again to a portion of the pattern. Since this portion with adhered material might become a opaque defect depending on the size thereof, this portion will have to be pattern-corrected at a later stage. Thus an etching method is needed by which the material adhered can be suppressed as much as possible.
A gas-assist etching is suggested as a method, which solves the problem of material adhertion as described above. The gas-assist etching is a method in which the etching is performed using a focused ion beam, while material to be etched and gas which is liable to chemically react are supplied at a suitable level thereof in the vicinity of the portion to be etched. Though this etching can suppress the material adhered to a certain degree; it does not suppress it completely. Moreover, though the gas to be supplied has a low etching capacity, the gas corrodes the material to be etched. Thus it is difficult to appropriately control the shape of a cross section of a portion to be processed.
Since these defects in the reticles also occur in photomasks for use in ultraviolet light exposure, a reticle repair device is used for correcting the defects of photomasks. In this reticle repair device, the opaque defect is corrected using an ion beam etching method, by locally removing the chrome serving as a shielding material. A clear defect is corrected using the method of Focused Ion Beam (FIB)-Induced Deposition using a focused ion beam, by locally coating a carbon film. Moreover, it is considered reasonable that when a mask is used in the X-ray equi-multiple exposure method, or EUV lithography, which is in the course of being developed both opaque defect repair and clear defect repair can be accomplished by a focused ion beam.
In a scattering stencil type reticle used in electron beam reduction projection lithography with an electron beam serving as an exposure source, we are faced with the following new problems, which do not exist in the conventional reticles:
(1) Since the reduction projection lithography method using an electron beam concerns a technology forming a pattern of less than 0.1 xcexcm, it is necessary to form an electron scattering material pattern of less than 0.4 xcexcm on the reticle in the event that the reduction factor is one forth. On the other hand, as the electron beam scattering material requires a thickness of approximately 2 xcexcm, the aspect ratio of the electron beam scattering material pattern is approximately 5. Since severe specification is required for the size of a portion to be processed in a reticle and a cross sectional shape thereof, the miniaturization processing for the reticle requires an advanced processing technique as the aspect ratio increases, thus its processing becomes excessively difficult to carry out.
(2) The reticle, or mask, used in reduction projection lithography using an electron beam is constituted by the electron beam scattering material alone, and is of so-called scattering stencil type. Namely, in this scattering stencil type mask, a pattern of an electron beam scattering material is not formed on the substrate which is illuminated by the electron beam, instead, the region which is illuminated by the electron beam is opened, so that the projected pattern is formed and consists of the electron beam scattering material alone. In scattering stencil reticle, incident beam is usually absorbed by the membrane or substrate but can pass through void, aperture, opening which forms a pattern. In scattering stencil reticle, incident beam is scattered by the membrane or substrate but can pass through void, aperture, opening which form pattern to be projected. In scattering membrane reticle, incident beam is scattered by the scatterer on the reticle surface, which forms pattern to be projected and almost pass through membrane. In a case where the electron beam scattering material is formed on a substrate, to form the film of the electron beam scattering material selectively for the purpose of correcting a clear defect is relatively easy. However, for the scattering stencil type reticle, which does not require the substrate, the electron beam scattering material must be accumulated in the horizontal direction from the clear defect so that the clear defect can be modified. A method of modifying a clear defect of a scattering stencil type reticle has not been established in the prior art.
(3) With conventional repair method using FIB, the inclination of repaired wall or side face is 86xc2x0 to 87xc2x0. But for EBPS using scattering stencil mask, the inclination of the wall or side face of void, or opening, or aperture should be within 90xc2x0xc2x11xc2x0.
(4) For repair of clear defects, suitable materials, which scatter electrons as the membrane does and does not absorb electrons, should be supplied to the defects. So far such suitable materials with proper structure and the method for providing such materials are not selected or established.
In a mask used for the reduction projection method by a highly accelerated electron beam, which the inventor of the present invention proposed, two conditions are required. One is that the electron scattering portion scatters the electrons in an appropriate manner, and another is that the electron scattering portion does not absorb the electrons and that heat is not generated significantly due to irradiation of the electron beams. It is required that the electron scattering property of the substance formed selectively in the clear defect portion be the same as in other portions of the mask when electron beam reduction projection lithography is applied. Further, it is desirable that the irradiated electron beams are accelerated to more than 50 kV in order to suppress electron absorption into the electron scattering substance of the mask. The following explains the effect of absorption of electrons into the electron scattering substance of the mask. When electrons have been absorbed in the membrane, heat is generated in the portion of the membrane according to the absorption volume and the thermal conductivity. Thermal expansion is caused in that portion of the membrane, and, as a result, the pattern size is altered. For example, it is required that the allowable width of pattern fluctuation should be less than 12 nm for 0.10 xcexcm mode processing equivalent to 16 G-bit DRAM, for example, and the temperature-rise range was less than 0.1xc2x0 C. in the case where the mask material is, for example, a single silicon crystal.
As a material satisfying these conditions, a material including carbon and a material including silicon, or similar, may be used. However, since the electron scattering property of the material including carbon is somewhat deteriorated, the material including silicon is considered especially preferred. Specifically, materials such as silicon, where a boron or phosphorus is doped in an appropriate manner, or a silicon nitride, or silicon carbide are preferable. Thus, it is necessary to find an inert gas, which has superior processing characteristics relative to these mask materials.
The present invention has been made in view of these points, and it is an object thereof to provide a method to modify, or repair a mask with high scattering and low absorption.
In order to solve the above-described problems according to the first embodiment of the present invention, a method to modify a defect caused at the time of manufacturing a mask, having a pattern exposure portion comprising an electron beam scattering material, which is used when a reduction projection exposure image is projected onto a substrate to be projected, is proposed.
An ion entered into an electron beam scattering material for the purpose of modifying an opaque defect pattern is obtained by sputtering of scattering material atoms or molecules, or the cluster. During the sputtering, some of the atoms, molecules or cluster are gasified, while some are scattered in the molten state, proportional to the ion energy applied. Those, which were scattered in the molten state, are deposited in the vicinity of the etching portion, and those, which were gasified, are accumulated in the vicinity of a processing portion and might be deposited again. This phenomenon is called re-accretion formation, or re-deposition. As a result of diligent research done by inventor of the present invention, it was confirmed that this re-accretion formation can be suppressed by a gas-assist etching method, and further, that correction of an opaque defect pattern is possible when utilized in the modification of pattern defects in EBPS-use scattering stencil type mask. Moreover, the gas-assist etching method is a publicly known processing method, and for instance, is introduced as a review in Jpn J. Apply. Phys. Vol. 33 (1994) pp. 7094-7098 by L. R. Harriott.
Though the gas-assisted etching for a single crystal silicon or amorphous silicon is known, and the reactive gas is of course disclosed in the prior art, these types of gases or conditions are not utilized directly in the material for scattering stencil type mask of the reduction projection exposure method using the high acceleration electron beam of the present invention. This is because said scattering stencil type masks require a finest processing resolution of 0.4 xcexcm and a CD accuracy of 0.013 xcexcm or less.
In the above description, use of 100 nm-lithography was supposed. However, the exposure method of the present invention can provide the lithography process with up to 70 nm and 50 nm. Thus, as a natural consequence, the minimum processing resolution on the mask is required to be 0.28 xcexcm and 0.20 xcexcm and, thus, a CD accuracy should be less than 0.010 xcexcm. For the above reasons, it is not known whether or not the gas-assisted etching method utilizing the above-described gas can be applied to modify the pattern defects of a scattering stencil type mask for use with the EBPS, requiring a processing resolution of 0.4-0.2 xcexcm.
As a result of further research, the inventor of the present invention has found that, by supplying a corrosive gas at the time of gas-assisted etching to the vicinity of a processing portion against a substrate to be processed under an appropriate condition, a processing with a resolution of 0.4 xcexcm can be achieved. The inventor also found that applying this to modification of an opaque defect pattern of a scattering stencil type mask is very effective. Namely, as a gas to be supplied to the vicinity of processing portion by the method of gas-assisted etching, a type of gas, which is corrosive by adhering to an electron scattering material membrane on the mask, is preferable
The method to modify the scattering stencil type mask is characterized in that a charged particle beam is irradiated to said defect while a gas comprising at least a compound material including a halogen is being supplied, so as to eliminate said defect. By using this method, a high process resolution can be obtained. A high quality scattering stencil type mask for use with electron beam reduction projection system can therefore be produced.
In the method of repairing the scattering stencil type mask, it is desirable that said gas is the one that operates as an etching agent for silicon. Moreover, it is desirable that said gas includes one of the members of the following group: iodine, chlorine, xenon fluoride, carbon tetrachloride group, such as CCl4 and C2Cl4, carbon trichloride group such as CHCl3, carbon tetrafluoride group, such as CF4, or carbon trifluoride group such as CHF3. It is also desirable that said gas is supplied with a flow rate thereof being adjusted after having been diluted by an inert gas.
In another embodiment of the present invention, a method to repair a defect, which has been made when a mask is used for projecting an image onto a wafer by a reduction projection exposure, is proposed. The repair method forms a membrane in a defective portion composed of material, the main element of which is silicon or carbon, comprising substances containing at least one atom of nonmetal Vb group atoms and semiconductor atoms except silicon, nonmetal atom, and metal atom. Thus, it is possible to form a repaired membrane in which the electron scattering property is high and the absorption of the electron beam is comparatively low. Further, in a repair method of the above-described mask, it is desirable that the membrane enables forward scattering of an electron beam greater than 10 mrad. Thus, it may be verified that the membrane has the electron scattering property. Further, it is desirable that the number of electrons which are forward-scattered within an electron beam irradiation angle range of 10 mrad is less than 1.5% of a total number of irradiated electrons when an electron beam is irradiated to the membrane. It is desirable that the composition quantity ratio for containing at least one atom of the nonmetal Vb group atoms and the semiconductor atoms except silicon, the nonmetal atom, and the metal atom in the substance is less than 92% of all atoms. In this repair method, it is also desirable that the membrane includes phosphorous, and the composition quantity ratio of phosphorous is less than 92% of the total number if atoms. Further, it is desirable that the membrane includes titanium, and the composition quantity ratio of titanium is less than 73% of all atoms. Moreover, it is desirable that the membrane includes gold, and the composition quantity ratio of gold is less than 20% of all atoms. Still further, it is desirable that the membrane includes platinum, and the composition quantity ratio of platinum is less than 15% of all atoms.
The membrane may also be created by supplying a mixture of substances to the defective portion by mixing plural gasified substances comprising the electron beam scattering substances at a predetermined mixture ratio and using a focused ion beam, an electron beam, or an infinitesimal plasma for exposure. According to the above described composition, plural materials comprising electron beam scattering materials are mixed at a predetermined mixture rate; the mixture of plural materials is supplied to the defective portion, and the membrane is selectively formed in the defective portion by arbitrarily selecting one exposure source of a focused ion beam, an electron beam, or an infinitesimal plasma. Using this method, the electron scattering property is adjusted. Thus, a mask having high electron scattering property and low electron beam absorption may be obtained.
As explained above, a FIB-Induced Deposition method is regarded as the most likely selective membrane forming method for clear defect repair. The density of the selective deposition membrane the main element of which is carbon or silicon is, however, lower than the density of a reticle bulk (single silicon crystal), thus the electron scattering property is significantly lower than that of the electron scattering material of the reticle. Under these circumstances, it is necessary to obtain a membrane for a mask with a high electron scattering property and comparatively low electron absorption. Another embodiment of the present invention provides a method by which to repair a defect that has been made when a mask has been used to project a reduction exposure image onto a wafer in which a pattern projection portion composed of a void in membrane which scatters electrons has been included. The method includes a step of forming a membrane composed of a material, a main element of which is carbon or silicon, in the defective portion, and a step of forming a repair reinforcement layer on the membrane. According to this embodiment, a membrane composed primarily of carbon or silicon suppresses the electron absorption, and the repair reinforcement layer reinforces electron beam scattering. Thus, a mask with a high electron beam scattering property and low absorption of the electron beam may be produced. It is preferable that the repair reinforcement layer is composed of substances containing semiconductor atoms (except silicon), non-metal atoms, or metal atoms.
Further, the present invention provides a structure by which to repair a defect that has been made when a mask has been used to transfer a reduction projection exposure image to a wafer having a void, or opening in electron beam scattering substance for a pattern projection portion; and a method by which to repair a scattering stencil type mask including a membrane formed in the defective portion and composed of a substance, the main element of which is carbon or silicon.
The repair membrane is a layered structure, layer upon layer, a repair reinforcement layer being composed of substances containing semiconductor atoms (except silicon), nonmetal atoms, or metal atoms and formed at a suitable thickness on the membrane, the main element of which is carbon or silicon. The thickness of the membrane can be set freely, in order to minimize the mean free path of the electrons. If the elements, such as semiconductor except silicon, non-metal atoms, or metal atoms contained therein are different, the membrane conditions, such as the optimum thickness, changes. Further, when the electron scattering property of the membrane, the main element of which is silicon or carbon, is different, the optimum atom composition ratio of the layer containing at least the above described atoms is different. The metal atoms are mostly from organic metal material as a material source. In this case, carbon or hydrogen is also contained to a degree in addition to metal.